About pixel dimensions and printed
image resolution

Pixel dimensions measure the total number of
pixels along an image’s width and height. Resolution is the fineness
of detail in a bitmap image and is measured in pixels per inch (ppi).
The more pixels per inch, the greater the resolution. Generally,
an image with a higher resolution produces a better printed image quality.

Same image at 72‑ppi and 300‑ppi; inset zoom 200%

Unless an image is resampled (see Resampling),
the amount of image data remains constant as you change either the
print dimensions or resolution. For example, if you change the resolution
of a file, its width and height change accordingly to maintain the
same amount of image data.

In Photoshop, you can see the relationship between image size
and resolution in the Image Size dialog box
(choose Image >Image Size). Deselect
Resample Image, because you don’t want to change the amount of image
data in your photo. Then change width, height, or resolution. As
you change one value, the other two values change accordingly. With
the Resample Image option selected, you can change the resolution,
width, and height of the image to suit your printing or on-screen
needs.

File size

The file size of an image is the digital
size of the image file, measured in kilobytes (K), megabytes (MB),
or gigabytes (GB). File size is proportional to the pixel dimensions
of the image. Images with more pixels may produce more detail at
a given printed size, but they require more disk space to store
and may be slower to edit and print. Image resolution thus becomes
a compromise between image quality (capturing all the data you need)
and file size.

Another factor that affects file size is file format. Because
of the varying compression methods used by GIF, JPEG, PNG, and TIFF
file formats, file sizes can vary considerably for the same pixel
dimensions. Similarly, color bit-depth and the number of layers
and channels in an image affect file size.

Photoshop supports a maximum
pixel dimension of 300,000 by 300,000 pixels per image. This restriction
places limits on the print size and resolution available to an image.

About monitor resolution

Your
monitor’s resolution is described in pixel dimensions. For example,
if your monitor resolution and your photo’s pixel dimensions are
the same size, the photo will fill the screen when viewed at 100%.
How large an image appears on‑screen depends on a combination of
factors—the pixel dimensions of the image, the monitor size, and
the monitor resolution setting. In Photoshop, you can change the
image magnification on‑screen, so you can easily work with images
of any pixel dimensions.

A 620‑ by 400‑pixel image displayed on monitors of various
sizes and resolutions.

When preparing images for viewing on‑screen, you should consider
the lowest monitor resolution that your photo is likely to be viewed
on.

About printer resolution

Printer
resolution is measured in ink dots per inch, also known as dpi.
Generally, the more dots per inch, the finer the printed output
you’ll get. Most inkjet printers have a resolution of approximately
720 to 2880 dpi. (Technically, inkjet printers produce a microscopic
spray of ink, not actual dots like imagesetters or laser printers.)

Printer resolution is different from, but related to image resolution.
To print a high quality photo on an inkjet printer, an image resolution
of at least 220 ppi should provide good results.

Screen frequency is
the number of printer dots or halftone cells per inch used to print
grayscale images or color separations. Also known as screen
ruling or line screen, screen frequency is measured
in lines per inch (lpi)—or lines of cells per inch in a halftone
screen. The higher the resolution of the output device, the finer (higher)
a screen ruling you can use.

The
relationship between image resolution and screen frequency determines
the quality of detail in the printed image. To produce a halftone
image of the highest quality, you generally use an image resolution
that is from 1.5 to at most 2 times the screen frequency. But with
some images and output devices, a lower resolution can produce good
results. To determine your printer’s screen frequency, check your
printer documentation or consult your service provider.

Note:

Some imagesetters and 600‑dpi laser printers use screening
technologies other than halftoning. If you are printing
an image on a nonhalftone printer, consult your service provider
or your printer documentation for the recommended image resolutions.

Determine a suggested resolution
for an image

If you plan to print your image using a
halftone screen, the range of suitable image resolutions depends
on the screen frequency of your output device. Photoshop can determine
a recommended image resolution based on the screen frequency of
your output device.

Note:

If your image resolution is more than
2.5 times the screen ruling, an alert message appears when you try
to print the image. This means that the image resolution
is higher than necessary for the printer. Save a copy of
the file, and then reduce the resolution.

Choose Image > Image
Size.

Click Auto.

For Screen, enter the screen frequency for the output
device. If necessary, choose a different unit of measurement. Note
that the screen value is used only to calculate the image resolution,
not to set the screen for printing.

For Quality, select an option:

Draft

Produces a resolution that is the same as the screen
frequency (no lower than 72 pixels per inch).

Resampling

Resampling is
changing the amount of image data as you change either the pixel dimensions
or the resolution of an image. When you downsample (decrease
the number of pixels), information is deleted from the image. When
you resample up (increase the number of pixels, or upsample),
new pixels are added. You specify an interpolation method
to determine how pixels are added or deleted.

Resampling pixels

A.
Downsampled B.
Original C.
Resampled
up (selected pixels displayed for each set of images)

Keep
in mind that resampling can result in poorer image quality. For
example, when you resample an image to larger pixel dimensions,
the image loses some detail and sharpness. Applying the Unsharp
Mask filter to a resampled image can help refocus the image details.

You
can avoid the need for resampling by scanning or creating the image
at a sufficiently high resolution. If you want to preview the effects
of changing pixel dimensions on‑screen or to print proofs at different
resolutions, resample a duplicate of your file.

Photoshop resamples images using an interpolation
method to assign color values to any new pixels based on
the color values of existing pixels. You can choose which method
to use in the Image Size dialog box.

Nearest Neighbor

A fast but less precise method that replicates the pixels
in an image. This method is for use with illustrations containing
edges that are not anti-aliased, to preserve hard edges and produce
a smaller file. However, this method can produce jagged effects,
which become apparent when you distort or scale an image or perform
multiple manipulations on a selection.

Bilinear

A method that adds pixels by averaging the color values of
surrounding pixels. It produces medium-quality results.

Bicubic

A slower but more precise method based on an examination
of the values of surrounding pixels. Using more complex calculations,
Bicubic produces smoother tonal gradations than Nearest Neighbor
or Bilinear.

Bicubic Smoother

A good method for enlarging images based on Bicubic interpolation
but designed to produce smoother results.

Bicubic Sharper

A good method for reducing the size of an image based on Bicubic
interpolation with enhanced sharpening. This method maintains the detail
in a resampled image. If Bicubic Sharper oversharpens some areas
of an image, try using Bicubic.

You can specify a default interpolation method
to use whenever Photoshop resamples image data. Choose Edit >
Preferences > General (Windows) or Photoshop >
Preferences > General (Mac OS), and then choose
a method from the Image Interpolation Methods menu.

Change pixel dimensions of an image

Changing an image’s pixel dimensions affects
not only its on‑screen size but also its image quality and its printed
characteristics—either its printed dimensions or its image resolution.

Choose Image > Image
Size.

To maintain the current ratio of pixel width to pixel
height, select Constrain Proportions. This option automatically
updates the width as you change the height, and vice versa.

Under Pixel Dimensions, enter values for Width and Height.
To enter values as percentages of the current dimensions, choose
Percent as the unit of measurement. The new file size for the image
appears at the top of the Image Size dialog
box, with the old file size in parentheses.

Make sure that Resample Image is selected, and choose
an interpolation method.

If your image has layers with styles applied to them,
select Scale Styles to scale the effects in the resized image. This
option is available only if you selected Constrain Proportions.

When you finish setting options, click OK.

For best results when you produce a smaller
image, downsample and apply the Unsharp Mask filter. To produce
a larger image, rescan the image at a higher resolution.

Change the print dimensions and
resolution

When
creating an image for print media, it’s useful to specify image
size in terms of the printed dimensions and the image resolution.
These two measurements, referred to as the document size,
determine the total pixel count and therefore the file
size of the image; document size also determines the base
size at which an image is placed into another application. You can
further manipulate the scale of the printed image using the Print
command; however, changes you make using the Print command affect
only the printed image, not the document size of the image file.

If
you turn on resampling for the image, you can change print dimensions
and resolution independently (and change the total number of pixels
in the image). If you turn off resampling, you can change either
the dimensions or the resolution—Photoshop adjusts the other value
automatically to preserve the total pixel count. For the highest
print quality, it’s generally best to change the dimensions and
resolution first, without resampling. Then resample only as necessary.

Choose Image > Image Size.

Change the print dimensions, image resolution, or both:

To change only the print dimensions or
only the resolution and adjust the total number of pixels in the
image proportionately, select Resample Image and then choose an
interpolation method.

To change the print dimensions and resolution without
changing the total number of pixels in the image, deselect Resample
Image.

To maintain the current ratio of image width to image
height, select Constrain Proportions. This option automatically
changes the width as you change the height, and vice versa.

Under Document Size, enter new values for the height
and width. If desired, choose a new unit of measurement. Note that
for Width, the Columns option uses the width and gutter sizes specified
in the Units & Rulers preferences.

For Resolution, enter a new value. If desired, choose
a new unit of measurement.

To restore the initial values displayed
in the Image Size dialog box, hold down Alt (Windows)
or Option (Mac OS), and click Reset.

What affects file size?

File size depends on the pixel dimensions of an image
and the number of layers it contains. Images with more pixels may
produce more detail when printed, but they require more disk space
to store and may be slower to edit and print. You should keep track
of your file sizes to make sure the files are not becoming too large
for your purposes. If the file is becoming too large, reduce the
number of layers in the image or change the image size.

You can view the file size information for an image at the bottom
of the application window.